1. Field of the Invention
The present invention relates to a sleeve monopole antenna, and more specifically, to a full band sleeve monopole antenna with equivalent electrical length.
2. Description of the Prior Art
Digital multimedia applications such as MP3 player, satellite broadcasting, and Hi-Fi digital broadcasting have extended the application from personal usage with portability to mobile application due to a higher demand for a comfortable, digitalized driving environment of mobile industry. Therefore, today the broadcasting system in a mobile is more about receiving multimedia signals from different multimedia equipments than just receiving broadcasting signals from radio frequency modulation (FM) signals. To fit in to the prior art mobile FM radio system, more and more digital multimedia applications have built-in FM transmitter so that the music in digital form can be transformed into FM signals and transmitted to the mobile FM radio system.
A prior art FM antenna transmits FM signals with mono-frequency or with high transmission power and has small size to be carried and easily disposed on the vehicle. Considering the bandwidth of the signals with return loss less than −10 dB, such kind of FM antenna commonly has effective bandwidth of 2˜5 MHz and is not suitable for mobile FM radio system. It is therefore a convenient advancement that the FM transmitter has the ability to transmit signals with full bandwidth (88˜108 MHz) and the FM antenna has corresponding feature of transmitting signals with 88˜108 MHz bandwidth. The early FM antenna with 20 MHz bandwidth is accomplished by a monopole antenna with ¼ wavelength (about 75 cm) accompanied by a large ground end, for example, a ground end with area larger than 2 wavelength square, or a sleeve monopole antenna with length about 100 cm. However, the antennas above are too large to be installed on a vehicle.
Conforming to transmission regulations on FM bandwidth by Federal Communications Commission (FCC), the FM radiator of the FM transmitter must be placed as close as possible to the FM receiver because of the restriction of transmission power. Generally the FM receiver of the mobile FM audio system is disposed at the tail of a vehicle and the FM radiator is disposed at the rear window by connecting a 3-meter coaxial cable, which is buried under the seats or the carpet for outlook reason. Most FM radiators can be classified into two types: chip antenna or a 30 cm copper wire wrapped on a ferrite core collocating with the coaxial cable. Either type has a small size but the bandwidth of transmission is narrow and not uniform. For frequency sections that have impedance mismatching, part of the power reflects back to the coaxial cable when transmitted by the FM transmitter to the FM radiator through the coaxial cable. The reflected power is transmitted by the copper screen again but is shielded by the body of the vehicle, which brings waste of power to the FM radiator.
Please refer to FIG. 1. The sleeve monopole antenna 10 according to the prior art comprises a radiator 12 and a sleeve 14 (for grounding). The radiator 12 has a length of ¼ wavelength and the sleeve 14 provides route for the inverse phase signals of the radio signals. In other word, the sleeve monopole antenna 10 is a transformation of a dipole antenna and the sleeve 14 provides impedance matching and collaboration of the bandwidth for the radiator 12. To convert the phase of the signals, the length L of the sleeve 14 and the distance Rx between the sleeve 14 and the radiator 12 are important factors where the input impedance of the sleeve monopole antenna 10 depends on Rx and L determines the phase of the signals. It is a common practice to set the length L of the sleeve 14 as ⅛ to ¼ wavelength to provide signals with phase 180 degree. Furthermore, the sleeve 14 functions as a balun (balance-unbalance converter) to convert the one-way unbalanced signals into two-way out-of-phase signals where one way for the radiator 12 and the other way for a ground plane large enough or another radiator with ¼ wavelength.
Please refer to FIG. 2. A printed sleeve monopole antenna 20 according to the prior art winds the radiator 22 to reduce the dimension. However, the overall size of the prior art sleeve monopole antenna 20 cannot be further minimized since the length L of the sleeve 24 remains between ⅛ to ¼ wavelengths that is why the prior art antenna 20 has difficulty to be implemented on mobile FM broadcasting system.